Hello I'm a fan of Kingdom Hearts in Spain and Spanish forums moves the following theory.
In the credits as the characters come out with lyrics that say This leads to Kingdom Hearts but many theorize that when Sora and Riku go through a white background are in Roman numerals III.
This would mean, This leads to Kingdom Hearts III. Do you think?
They have also left the sentences ending the secret ending:
The past is reawakened as a new number.
In detail never seen before.
Prepare for awakening the next number